As large scale integrated circuits (LSI) become more complex and achieve higher operating speeds, a demand for finer pattern rules is emerging. Using ordinary photoexposure procedures, however, practical limits of resolution defined by the wavelength of the light source used are rapidly being approached. By photoexposure to g rays (436 nm) or i rays (365 nm) as the light source, for example, this limit is a pattern rule of approximately 0.5 .mu.m permitting manufacture of an LSI corresponding to a degree of integration required by a 16M bit DRAM. LSI prototypes have however already reached this stage, and there is thus an urgent need to develop still finer patterning methods.
In this context, far UV lithography appears to offer promise as a new patterning technique. This type of lithography permits patterning down to 0.3 to 0.4 .mu.m, and a pattern having effectively vertical walls with respect to the substrate can be obtained if a resist having a low light absorption is used. Moreover, as this technique makes it possible to transfer a pattern in one operation, it offers a higher throughput than electron beam lithography. High intensity KrF exima lasers are now being used as light sources for far UV radiation, but if they are to have practical utility in mass production techniques, a highly sensitive resist with low light absorption is required.
Recently, resists which are capable of chemical amplification with an acid as a catalyst (such as that proposed by Liu et al in J. Vac. Sci. Technol., Vol. B6, p 379 (1988)) have been shown to possess excellent properties such as sensitivity similar to or greater than conventional resists, high resolution and good tolerance to dry etching, and they are therefore regarded as being particularly promising insofar as concerns far UV lithography. Shipley Inc. is already marketing a negative resist that uses chemical amplification (SAL601ER7) consisting of three components, i.e. novolak resin, a melamine compound and an acid-producing agent, but a positive type resist using chemical amplification was not commercially available. However, although negative resists can be used for some LSI manufacturing processes such as wiring or gate forming, fogging tends to occur when they are used to form contact holes. Fine patterning is difficult, and positive resists are far more suitable for this purpose.
A high performance positive resist is, therefore, strongly desired. Ito et al have developed such a positive type resist that uses chemical amplification by adding an onium salt to a resin known as PBOCST wherein the OH groups of polyhydroxystyrene are protected by t-butoxycarbonyl groups (hereinafter, abbreviated as tBoc groups).
The onium salt used contains antimony as a metal component (Polymers in Electronics, ACS Symposium Series. No. 242 (American Chemical Society, Washington D.C., 1984),p. 11). In order to avoid contaminating the substrate, metal components in the resist material are generally undesirable, and the aforesaid PBOCST resist is therefore unsuitable from the process viewpoint. Ueno et al report a far UB positive resist having p-styreneoxy tetrahydropyranyl as the principal component to which an acid-producing agent is added (36th Oyoo Butsuri Gakkai Kanren Rengo Koenkai, 1980, 1p-k-7).
The inventors however found that this type of resist tended to undergo positive to negative inversion when exposed to far UV radiation, electron beams or X-rays. With a two-component positive resist system consisting of a resin wherein the OH groups are protected by protecting groups combined with an acid-producing agent, many protecting groups have to be decomposed so that the resist can dissolve in the developing solution, and there is a considerably high risk of film thickness variations, in-file stress or air bubbles.
However, using a three component system with more diversified functions as a positive resist which makes use of chemical amplification, i.e. an alkali-soluble resin, a solution blocking agent and an acid-producing agent, not such acid is required to decompose the solution blocking agent. This suggests the possibility of reducing the aforesaid film thickness variations and generation of air bubbles so that the resist is more useful for very fine patterning applications. Hechist Inc. has developed a resist for X-ray lithograph, RAY/PF, wherein an acetal compound as a solution blocking agent is added to novolak resin, and an acid-producing agent is then added.
RAY/PF undergoes chemical amplification at room temperature. and its resist sensitivity closely depends on the time for which it is left after exposure to X-rays. In actual practice, strict regulation of the time between the exposure and developing processes is not easy. It might therefore be inferred that using this material, it is difficult to control pattern dimensioning. Further, its light absorption at the exposure wavelength of a KrF exima laser (248 nm) is too high.
In general, in order to make use of chemical amplification, many resists required post-exposure backing (PEB). Compared to resist systems where the system is merely left at room temperature to undergo chemical amplification, an additional resist process is involved, but as it is not necessary to regulate the time between exposure and developing so strictly, it is easier to control the properties of the resist.
In systems where hydrolysis occurs in the chemical amplification process, water is required for the hydrolysis reaction, and the resist must therefore contain a large amount of water.
In general, an organic solvent which is immiscible with water such as thoxyethyl acetate is used as a coating solvent for the resist material, and in many resist materials, the resin itself is not very compatible with water. It is also difficult to blend a predetermined amount of water with these resist systems, and even if the blending can be achieved, the number of components that must be controlled is larger which renders the system more complex. The decomposition of tBoc groups, however, proceeds with only two components, i.e. the tBoc group and an acid catalyst. As water is not required as a third component, the reaction is simpler and is well suited to chemical amplification.
It is known that the tBoc derivatives of many compounds block the dissolution of novolak resin, and that the tBoc group is useful for conferring insolubility. Schlegel et al reported a three component positive resist comprising novolak resin, a solution blocking agent consisting of the tBoc derivative of bisphenol A, and pyrogallol methanesulfonic acid ester (37th Oyoo Butsuri Gakkai Kanren Rengo Koenkai, Spring 1990, 28p-ZE-4).
Schwalm et al developed a bis (p-t-butoxycarbonyloxy-phenyl) iodinium hexafluoroantimonate as a combined solution blocking agent and acid-producing agent. (Polymer for Microelectronics, Tokyo 1989, Session A38) which, when mixed with novolak resin, is used as a positive resist with far UV light. However, as this system contains a metal, and as the light absorption of the novolak resin is too high, it is not suitable for practical application.
Moreover, if patterning was performed on conventional positive resists that undergo chemical amplification by means of far UV light, electron beams or X rays, the pattern tended to suffer from overhang.
This is assumed to be due to the fact that the solubility of the resist surface decreases (K. G. Chiong et al, J. Vac. Sci. Technol., Vol. B7, (6), p. 1771, (1989)). Overhang makes pattern dimension control difficult and impairs dimensional controllability even when substrates are patterned by dry etching.
Further, the base of the pattern was etched too fine, and easily led to pattern collapse.
As described hereintofore, many positive resists capable of chemical amplification which have a novolak or polyhydroxystyrene base resin and are sensitive to far UV radiation, electron beams or X rays, have been reported in the art. All of them are however associated with problems, and it is still difficult to use them in practical applications.